Выдержка из файла конфигурации /etc/network/interfaces :

auto tun0 
iface tun0 inet static 
    address 172.18.1.2 
    netmask 255.255.255.0 
    pre-up ip tunnel add mode ipip tun0 local 172.19.1.22 remote 172.19.1.206
    post-down ip tunnel del mode ipip tun0 local 172.19.1.22 remote 172.19.1.206

Конфигурация на Cisco:

interface Tunnel0 
ip address 172.18.1.1 255.255.255.0 
ip mtu 1476 
tunnel source 172.19.1.206 
tunnel destination 172.19.1.22 
tunnel mode ipip ip